OS: Windows/FreeBSD/Linux (CentOS, Debian, Ubuntu, etc.)
Virtualization: VmWare, HyperV
Networking: Firewall, NAT, VPN, DHCP, DNS, SSH/SCP, FTP, HTTP/HTTPS
Windows services: RDP/TS , IIS, AD, GP, NLB/Cluster
Mail services: MS Exchange, Postfix, Exim, sendmail, Kerio, antispam
Databases administration: MS SQL, MySql, PostgreSQL, interbase/firebird
Scripting: VBS, bash, PHP, perl